Optical isolators have a Faraday rotator that rotates the plane of polarization of incident light by applying a magnetic field, and in recent years, have come to be used in not only optical communications, but in laser processing machines as well.
Terbium-scandium-aluminum-garnet-type single crystals (TSAG: Tb3Sc2Al3O12) have conventionally been known as Faraday rotators used in such optical isolators (refer to the following Non-Patent Document 1).